State officials have said same-sex marriages could begin almost immediately after a decision.

McShane heard arguments on the case in April, and rejected an attempt by the National Organization for Marriage to intervene.

The group argued it should be allowed to defend the ban on behalf of its Oregon members after the attorney general's decision. But McShane said the attorney general is accountable to Oregon voters, not a third party.

Seventeen states and the District of Columbia have legalized same-sex marriage. Judges in seven other states have struck down bans on gay marriage, though officials are appealing. More challenges are pending.

The most recent was Tuesday, when U.S. District Magistrate Judge Candy Dale overturned Idaho's ban. On Thursday, a federal appeals court placed the ruling temporarily on hold while it decides whether to issue a lengthier stay while Idaho's governor and attorney general appeal.

In Oregon, Portland attorneys Lake Perriguey and Lea Ann Easton filed a lawsuit in October on behalf of two women in a relationship for more than 30 years. Two months later, the American Civil Liberties Union and lawyers from two firms went to court on behalf of a lesbian couple and a gay couple.

Oregon law has long prohibited same-sex marriage. Its ban, approved by 57 percent of voters, came months after Multnomah County briefly issued marriage licenses to same-sex couples. Multnomah includes Portland and is the state's largest county.

About 3,000 gay couples were allowed to marry before a judge halted the practice. The Oregon Supreme Court later invalidated the marriages.

Advocacy groups say they have collected enough signatures to ask voters in November whether same-sex marriage should be legal. They have said they would discard the signatures if the judge rules in their favor. The group has until July 3 to submit just over 116,000 valid signatures.